Roundup: spine-tingling whodunits; Death by Sheer Torture, by Robert Barnard. New York: Charles Scribner's Sons. 186 pp. $10.95. The Thomas Street Horror, by Raymond Paul. New York: The Viking Press. 303 pp. Unseemly End, by Roderick Jeffries. New York: St. Martin's Press. 199 pp. $9.95 . Ellery Queen's Maze of Mysteries, edited by Ellery Queen. New York: The Dial Press. 283 pp. $12.95.

Robert Barnard's Death by Sheer Torture contains some of the most eccentric characters ever to inhabit a mystery. One of them has killed poor Leo Trethowan -- by snipping the cord to the shut-off switch for the strappado, an ancient torture device, into which Leo had placed himself.

To make things stranger, the victim was wearing ''gauzy spangled tights'' at the time of his death. And even more bizarre, it is the victim's long-estranged son, Perry, who must solve this crime.

Possible suspects include: Sir Lawrence, Leo's senile brother; Aunt Kate, an admirer of Hitler; Aunt Sybilla, a prodigious gossip; and the children of Perry's unpleasant brother, Peter, who are known collectively as the ''Squealies'' and are released from their rooms only on select occasions.

Harpenden, the family house, is a Victorian structure built with the too-much-is-not-enough philosophy, and all the family members do their best to keep up with its considerable eccentricity.

Barnard presents the flaws and foibles of the Trethowan family with great wit , and ''Death by Sheer Torture'' is an entertaining country-house mystery gone loony.

Historical mysteries are proliferating these days, and a fine example is Raymond Paul's The Thomas Street Horror. Set in New York during the 1830s, it's about attempts to solve the sensational murder of a prostitute.

The heroes are Davy Cordor, a young reporter for The Sun, and Lon Quinncannon , a defense attorney whose ratiocinative powers are considerable. They are virtually the only people who believe the accused -- young dandy Richard Robinson -- is innocent.

Author Paul, who teaches a course in the history of the detective novel at Montclair State College, has worked hard to make the historical flavor accurate. The scenes of the newspaper world and the final courtroom scene are especially good.

Paul has also labored to capture the prose style of the early 19th century, and he does it well, with only an occasional slip.

Simon Bognor returns to Oxford, his old stomping ground, for a reunion of the Apocrypha College in A Small Masterpiece, only to find that the master of the college is poisoned at a dinner Bognor attends.

Bognor is assisted in his search for the murderer by the attractive motorcycle-riding Dr. Hermine Frinton; is put down as usual by Parkinson, his superior at the Board of Trade; and is given the customary hard time by Monica, now his wife.

''A Small Masterpiece'' is, like all of Tim Heald's Bognor mysteries, amusing , and the much-put-upon Bognor is an increasingly likable, if bumbling, solver of crimes.

Readers in search of shorter mysteries would enjoy Ellery Queen's Maze of Mysteries, which contains a short novel, three ''novelets,'' and 14 short stories. Authors include: Queen, Ruth Rendell, E. X. Ferrars, Michael Gilbert, and Lawrence Block. A readable assortment.

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